Superheated tropical ants at risk to climate change
STRI/DICYT The next time you’re traipsing through a tropical forest and feel tempted to complain about the heat, think of the ants. Since they’re so small they’re easily superheated on sunexposed surfaces from the ground to the canopy. The forest they experience can be 10-15 degrees Celsius warmer than ambient temperature, which puts them at risk of passing out from ant heat stroke.
A new study by a group of scientists working at STRI on Panama’s Barro Colorado Island explored the temperature extremes tolerated by 88 species of ants. On average, canopy dwelling ants lost motor control once their temperature hit 50 C. Forest-floor ants conked out, on average, at 46 C.
The study, published in Global Change Biology, addresses the thermal adaptation hypotheses, which “predicts limited acclimatization of thermal maxima to a warming world, with tropical species at the greatest risk of extirpation,” wrote the authors, led by Michael Kaspari of the University of Oklahoma.
Canopy-dwelling ants are already regularly exposed to potentially paralyzing heat. All but three species in the study faced the limit of the temperatures they tolerate under natural conditions. Few ground-dwelling ants were found to be frequently exposed to higher temperatures than they can tolerate.
To establish the low end of ant thermal tolerance, researchers put the ants in petri dishes surrounded by Cryopak gel packs. Some ants tolerated temperatures as low as 4 C. The highest maximum temperature tolerated by any species was 56 C.
“In global change biology how you measure temperature matters. Temperatures from weather stations may approximate a tapir’s thermal experience, but not that of ants (and other tiny things) crawling around in the sunshine,” said Kaspari.
Panama’s average temperatures are expected to rise between 2 and 5 degrees Celsius by 2080 but the authors don’t expect increased air temperatures alone to pressure the ants. Less cloud cover, a possible result of more intense and less frequent storms, could increase sun exposure, which may be the ants’ biggest risk from global change. Tree mortality due to drought could also increase the amount of forest floor exposed to direct sunlight.
“The future of tropical biodiversity in a warming world comes down to this: are tropical species and their interrelationships unusually fragile?” asked Kaspari. “Or is the tropics, as the engine of earth’s biodiversity, unusually robust?”